Electrical resonators are used in many applications. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are configured as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a bulk acoustic wave (BAW) resonator. Typically, there are two types of BAW resonators: a Film Bulk Acoustic Resonator (FBAR) and a solidly mounted bulk acoustic resonator (SMR). Both the FBAR and the SMR comprise acoustic stacks that are disposed over a reflective element. The reflective element of an FBAR is a cavity, normally in a substrate over which the acoustic stack is mounted. The reflective element of an SMR is a Bragg reflector comprising alternating layers of high acoustic impedance and low acoustic impedance layers.
BAW resonators have the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
Desirably, the BAW resonator excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., y-direction) of the piezoelectric layer.
Unfortunately, acoustic energy can be lost to regions of the BAW resonator structure that are outside the active area of the BAW resonator. This acoustic energy is manifest in various types of acoustic modes including, for example, so-called lateral modes, which have propagation vectors in a direction that are perpendicular to the direction of TE modes, the desired modes of operation. Among other adverse effects, lateral modes deleteriously impact the quality (Q) factor of an FBAR device. In particular, the energy of Rayleigh-Lamb modes is lost at the interfaces of the FBAR device. As will be appreciated, this loss of energy to spurious modes is a loss in energy of desired longitudinal modes, and ultimately a degradation of the Q-factor.
What is needed, therefore, is a BAW resonator structure that overcomes at least the shortcomings of known BAW resonators.